1. Field of the Invention
This invention relates to circuit breakers of the type having a trip motor energized by a trip circuit responsive to selected fault conditions producing overcurrents smaller in magnitude than short circuit currents. In particular, it relates to a circuit breaker that couples the magnetic flux generated by short circuit currents into the magnetic circuit of the trip motor to trip the circuit breaker open independently of energization of the trip motor coil by the trip circuit. Such an arrangement is particularly advantageous in providing an instantaneous trip function for subminiature circuit breakers, but can also be applied to larger circuit breakers.
2. Background Information
One use of subminiature circuit breakers is in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. As such, they are subjected to heavy use and therefore must be capable of performing reliably over many operating cycles. They also must be small to accommodate the high density layout of circuit breaker panels which make circuit breakers for numerous circuits accessible to a user. Subminiature circuit breakers can be used in an environment where they are subject to vibration. The circuit breaker must trip consistently within tolerance yet not be tripped out by vibration or shock loading.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I.sup.2 R heating resulting from the overcurrent. Some aircraft systems have also provided ground fault protection, but through the use of additional devices, namely current transformers which in some cases are remotely located from the protective relay. There is a growing interest in providing additional protection, and most importantly arc fault protection. Currently available subminiature circuit breakers do not respond to arc faults which are typically high impedance faults and can be intermittent. Nevertheless, such arc faults can result in a fire. Finally, there is an interest in providing an instantaneous trip in response to very high overcurrents such as would be drawn by a short circuit.
While larger circuit breakers, even the "miniature" circuit breakers used in residential applications provide multiple protection functions, the currently available subminiature circuit breakers do not have such combined features. Again, the challenge is to provide alternative protection in a very small package which will operate reliably with heavy use over a prolonged period. A device which meets all the above criteria and can be automatically assembled is desirable.